The properties of metals can be altered by processing at high temperatures where changes in microstructure, chemistry, and surface conditions can occur.
There are many different types of high temperature processes used for treatment of metals which include annealing, sintering, nitriding, carburizing and others. In this application, as an example steel is employed as the metal, although other types of metal could be processed.
In addition to high temperature, one of the common features of all of these processes is a specially controlled atmosphere. The atmosphere is designed specifically for the requirement of the process (carburizing, decarburizing, nitriding), but also prevents any form of oxidation. Thus the function of the atmosphere is to control a specific chemical reaction with the metal.
If steel is processed in air above 400° F., without the advantages of a special or protective atmosphere, oxidation of the surface will occur. In most situations, oxidation of the surface has a deleterious effect on properties and performance of steel, particularly when this occurs at temperatures above 1000° F.
There are several standard methods to create or generate a controlled atmosphere that can be used during thermal processing of steel:
1. pure gas supply from cylinders or tanks;
2. commercially pure gas supply from onsite generation plants, which include examples such as                a. cryogenic supply for nitrogen, oxygen, hydrogen, argon and helium,        b. pressure-swing absorption for nitrogen and oxygen,        c. membrane separation for nitrogen and oxygen, and        d. electrolytic separation for hydrogen and oxygen;        
3. catalytically assisted cracking of chemical feeds e.g. methane, ammonia for hydrogen; and
4. combustion of gas such as natural gas under controlled conditions to produce mixtures of CO, CO2, H2, H2O, and nitrogen—these atmospheres can be described as either exothermic or endothermic depending on the proportion of the components present and the carbon potential of the atmosphere (exothermic atmospheres remove carbon from steel while endothermic atmospheres add carbon to steel) (The words exothermic and endothermic refer to the thermodynamic conditions of the reaction where the free energy of the reaction is either positive or negative, as described by an Ellingham diagram).
The conventional equipment used to generate a standard exothermic or endothermic atmosphere are known as Exothermic Generators and Endothermic Generators. This stand-alone equipment typically comprises:
1. a nozzle or burner for mixture, ignition and combustion of air and a combustion gas where the mixture is directed into a chamber;
2. controls so that the mixture or proportion of air and a combustion gas may be kept within very precise conditions to produce repeatable and consistent mixtures of CO, CO2, H2, H2O, and nitrogen—there is a large body of theory and industrial practice that describes the complexities of this seemingly simple reaction—in steel-making literature, the interrelationship of these components is described as the “Water-Gas Reaction”—control of the temperature of reaction is also extremely important and equipment is most often provided to control the pressure input of the air and natural gas in order to control the temperature of the combustion;
3. the chamber into which the gas mixture is directed is usually water cooled externally, and may or may not include catalysts, heat exchange mediums, and filters—the combustion chamber may be vertical or horizontal;
4. a single or 2-stage heat exchanger and/or chiller in which water vapor can be removed from the combustion gas product—there is normally a filter included as well as drains to allow removal of the condensed water from the combustion atmosphere;
5. analytical equipment to monitor the combustion mixture;
6. a vent or by-pass stack that allows the products of combustion to be vented to atmosphere until such time as the combustion process is stable or the conditions in the actual furnace require addition of atmosphere; and
7. the stand-alone generator is usually rated or described based on the volume of combustion gas produced per hour.
Such stand-alone generators are usually characterized by:
1. relatively large use of cooling water which normally requires additional chilling, cooling, etc. for recirculation or, in the worst cases, direct discharge to a drain;
2. relatively high maintenance costs;
3. instability of the composition of the combustion gas mixture—very small differences in either temperature of combustion or proportion of air and natural gas can have a large effect on the products of combustion and, as a result, on the stability or lack of stability of the reaction from the atmosphere; and
4. relatively low costs when compared to the supply of pure gases, either directly or by generation.
Heating of the metal processing furnace may be achieved by electric resistance elements or by the combustion of natural gas within sealed burners. In most cases where the atmosphere in the metal processing furnace is required to perform a specific chemical reaction, other than simple heating, the atmosphere for combustion of the burners is completely separated from the special atmosphere used inside the metal processing furnace. This is because the combustion of air and gas for maximum heat generation in the burner provides an atmosphere composition that is not suitable for either exothermic or endothermic processing in the metal processing furnace.